An asset or task is made up of segments with translatable text, and attributes and segments with non-translatable markup. Selecting the Show Markup check box allows you to see the segments with non-translatable markup as well as those with translatable text and attributes. You cannot edit markup segments.
Here is an excerpt of an asset pair opened in the Browser Workbench with Show Markup off (the default).
Note that entry pair 5 is missing. Only 4 and 6 display here.
Here is the same excerpt with Show Markup turned on.
You now see the entry pair 5, which has no text, only markup. If the markup segment is extremely long, Browser Workbench displays only the beginning and end portions of the markup, with ellipsis in between.